A centerpiece of President Obama’s strategy for defeating the Islamic State is mobilizing tribal fighters to join the Iraqi military in retaking Anbar province and others dominated by Sunnis. But new research shows that the jihadists have been working since 2009 to gut the very Sunni tribal leadership on which Obama’s rollback depends — making the U.S. campaign much more difficult.

U.S. strategists want to create a “national guard” version of the tribal militia known as the Awakening, which in 2007 and 2008 crushed al-Qaeda in Iraq, the predecessor of the Islamic State. But overlooked evidence shows that the jihadists have worked systematically to destroy the Awakening and assassinate tribal leaders who might challenge their rule.

Despite these mea culpas, U.S. planners may be making a similar mistake in assuming that the tribal networks can be rebuilt quickly. U.S. officials believe that Sunni support has been galvanized by the removal of polarizing Prime Minister Nouri al-Maliki, a Shiite. That’s true, but fighting the jihadists will be a long uphill road.

Research documenting the Islamic State’s onslaught was compiled by Craig Whiteside, a former Army officer who fought in Iraq and now teaches at the Naval War College. By his count, at least 1,345 Awakening members have been killed in Iraq since 2009 by the Islamic State or its predecessor organizations. “In the Sunni areas where the Iraqi government had little control, it did not take long for the Islamic State to slowly and methodically eliminate resistance one person at a time,” he writes in a military blog called “War on the Rocks.”

Whiteside cites the example of the strategic town of Jurf al-Sakhar, south of Baghdad. Between 2009 and 2013, 46 Awakening members were killed in 27 different incidents there. The dead included four sheiks from the local Janabi tribe. Similar killings across Sunni areas of Iraq “were barely noticed by the Iraqi government or in the media,” Whiteside writes.

The jihadists documented their assassination campaign in a grisly video called “The Clanging of the Swords,” which Whiteside cites in his report. Watching the video, you see a series of drive-by assassinations, accompanied by heroic Islamic music, as Islamic State fighters gun down selected vehicles on the road or pedestrians on the streets. “The hungry lions chase their prey,” says an Arabic narrator, whose words are translated into English. It’s clear that the assassins’ intelligence is precise.

The Islamic fighters also targeted Iraqi police and army units in Sunni areas and Baghdad itself, starting more than two years ago. Islamic State communiques released in February 2013 claimed that in the second half of 2012, the group conducted 37 attacks in Baghdad and 43 assassinations in other areas of Iraq. U.S. analysts failed to see this gathering storm.

As its campaign against the Sunni tribal forces gained momentum in 2012 and 2013, the Islamic State began offering amnesty to Sunnis who had been part of the Awakening militia or the Iraqi security forces. The jihadist video shows scores of Sunnis experiencing “the joy of repentance” in an auditorium in Anbar. They recite a pledge of penitence together and then embrace masked jihadists on stage, one by one.

To swell its ranks further, the Islamic State staged a series of daring prison raids it called “Breaking the Walls.” Whiteside counts seven prison assaults between July 2012 and July 2013, culminating in a raid on Abu Ghraib prison that freed more than 500 senior Islamic State fighters, including one named Abu Wahib, who later became the group’s leader in Anbar. The importance of this prison-break campaign in the rapid build-out of Islamic State forces wasn’t understood by U.S. analysts.

U.S. officials argue that Sunni tribal leaders still want to work with American military advisers — all the more so after the jihadists’ brutal campaign of intimidation. As Sheik Zaydan al-Jibouri told me in Amman last month, “We want to create a strategic relationship with the Americans.”

But this time around, the tribal leaders must combat a deeply entrenched enemy. The Islamic State controls the ground; it has the intelligence; it has fierce, combat-hardened fighters. Obama is right to seek Sunni “boots on the ground” for the campaign against the jihadists, but he needs to explain better to the American public the roots of this conflict, and how difficult and protracted it will be.